Modulating reflector circuit

ABSTRACT

A modulating reflector circuit arrangement comprising a transistor configured by means of a feedback arrangement upon operation within a linear region of the transistor&#39;s current/voltage characteristic to reflect an incoming amplitude modulated signal with an increased amplitude and a modulator operable to modulate power to the transistor with a periodic waveform in which the reciprocal of the period of the periodic waveform is the required sideband frequency of the reflected signal and the waveform is selected such that the arrangement reflects a substantially single sideband signal.

BACKGROUND OF THE INVENTION

This invention relates to a circuit arrangement in particular, althoughnot exclusively, for use with a pseudo passive transponder (PPT).

Conventionally PPT's use a diode detector arrangement to detect anincoming signal from an interrogating source. The diode can be operatedas a modulated reflector by modulating the incoming signal withinformation and reflecting it back to the interrogating source. Thereflected signal is however of reduced amplitude with respect to theincoming signal which limits the useful range of the transponder. Inaddition the reflected signal has a double sideband which increases thecomplexity of the circuitry required at the interrogating source andreduces the spectral efficiency of the system. A need exists thereforefor a circuit arrangement for use in a PPT which can be made availableat relatively low cost, which can increase the available operatingrange, and preferably emit energy with only a single sideband.

SUMMARY OF THE INVENTION

This invention provides a circuit arrangement comprising a transistorconfigured to detect an amplitude modulated signal upon operation withinthe non-linear relatively low gain region of the transistorscurrent/voltage characteristic, the transistor also being configured bymeans of a feedback arrangement upon operation within a linearrelatively higher current and gain region of the characteristic toreflect the signal with an increased amplitude.

Surprisingly, the applicants have found that a single transistor can beused both as a so called "cold cathode" detector and a reflectiveamplifier merely by changing the operating conditions of the transistor.As compared to the known diode arrangement, there need not be asignificant increase in power consumption.

In a preferred embodiment, the transistor is a field effect transistor(FET) which is switchable between the detect and reflect modes byincreasing the drain/source current. With such a device, when operatedat microwave frequencies, the feedback can be provided via the intrinsiccapacitance between the gate and source of the FET.

According to another aspect of the present invention there is provided amodulated reflector circuit arrangement comprising a transistorconfigured by means of a feedback arrangement upon operation within alinear region of the transistor's current/voltage characteristic toreflect an incoming amplitude modulated signal with an increasedamplitude and modulating means operable to modulate power to thetransistor with a periodic waveform in which the reciprocal of theperiod of the periodic waveform is the required sideband frequency ofthe reflected signal and the waveform is selected such that thearrangement reflects a substantially single sideband signal.

BRIEF DESCRIPTION OF THE DRAWING

In order that the invention may be well understood, embodiments thereofwill now be described by way of example with reference to theaccompanying drawings, in which;

FIG. 1 is a schematic circuit diagram of a circuit arrangement;

FIG. 2 shows a typical voltage/current characteristic of a FET;

FIG. 3 shows the detected signal in another mode of operation; and

FIG. 4 is a circuit diagram of a circuit arrangement for use at UHFfrequencies.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 1 a circuit arrangement comprises a field effecttransistor 1, which can for example be either a silicone JFET or a GaAsdevice, depending on the required operating frequency. An antenna 2supplies a signal via a matching network 3 to the gate 1_(g) of thetransistor. The drain 1_(d) is connected via a bias network 4 to a powersupply 13 producing a rail voltage V_(d). The bias conditions on thegate side are set by means of an appropriate bias network 5 connected toa voltage rail V_(g). The source 1_(s) is connected via a bias network 6to a variable current source 7 which is itself connected to a controlcircuit 8. The bias networks 4, 5, 6 provide isolation between therelatively low voltage, low frequency signals on the control side, andthe typically much higher frequencies with which a transponder isdesigned to operate.

An output 9 is taken from the source side of the transistor from beyondthe bias network 6. Termination circuitry 11, 12 is provided on thedrain and source sides. The exact nature of the components will varydepending upon the frequency of operation, but essentially the networks11, 12 determine the frequency and phase characteristics of the circuit.

A feedback arrangement is provided by a line 10 and capacitor C₁. Atmicrowave frequencies it is possible by appropriate selection of thetermination circuitry 11, 12 to exploit the intrinsic capacitancebetween the gate 1_(g) and source 1_(s) to provide the feedbackcapacitance C₁.

The variable current source 7 can be used under control of controlcircuit 8 to alter the drain source bias current and thereby cause thecircuit to operate in one of three modes as will now be described withreference to FIG. 2. FIG. 2 shows the typical variation in drain sourcevoltage V_(ds) with drain current I_(d), the so-called transconductancecurve of a FET. With the transistor operating within the non linearrelatively low gain region A it can, as is well known, thereby act as adetector by amplifying higher amplitude signals relatively more thanlower amplitude signals. Increasing the drain current further thetransistor operates in the linear relatively higher current and gainregion B due to the feedback provided on line 10 and by capacitor C₁.The transistor then acts as a negative resistance or amplifierreflecting any incoming signals with an increased amplitude. Informationcan be transmitted to an interrogation source by modulating the drainsource current using the variable current source 7 and control circuit8. Unlike the prior art diode arrangements, the amplitudes of theinformation carrying sidebands of the reflected signal when operated asa modulated reflector can be greater than the incoming signal.

As with the prior art diode arrangement the present invention whenoperated as a modulated reflector produces a double sideband signal.Unlike the prior art arrangement a single sideband signal can however beproduced by modulating the power to the transistor 1 with a special formof periodic waveform. Referring to FIG. 1, modulating means 14 controlsthe power supply 13 and control circuit 8. The modulating means isconfigured to modulate either the drain source current and/or the railvoltage V_(d) with a complex periodic waveform, for example a distortedsine wave. The periodic waveform is selected such that the reciprocal ofthe period of the waveform is equal to the frequency of requiredsideband. It has been found that modulating the current and/or voltagein this way when in its reflective mode produces a single sidebandemission. This is because the sideband energy is generated from twoforms of modulation, phase and amplitude. The generated sideband pairsfrom the two forms of modulation have an opposite phase relationship andtherefore when the amplitude of the sidebands is balanced the sidebandson one side cancel whilst the others combine resulting in a singlesideband. Modulating the power to the transistor has the effect ofmodulating the phase and amplitude of the reflection coefficient of thetransistor. The concept of modulating the power to the transistor toproduce a single sideband emission is considered inventive in itself andaccordingly this concept is not restricted to a circuit arrangementwhich has to be operable both as a detector and as a modulatedreflector.

The circuit arrangement shown in FIG. 1 can also be operated as a superregenerative receiver further increasing the drain current I_(d) toregion C. In this way the transistor becomes unstable and willoscillate. FIG. 3 shows the drain source voltage V_(ds) varying withtime as the variable current source 7 is switched on at times t₀ t₂ andoff at t₁, t₃ by the control circuit 8. The amplitude of theoscillations increases up to a maximum level V_(ds)(max). The circuitcan be used as a sensitive detector due to the fact that an incomingsignal to the gate 1_(g) as long as it is at the frequency ofoscillation, will lead to a decrease in the rise time T₁, as compared tothe rise time T₂, to reach a given level of oscillation amplitude V_(ds)¹ when no such signal is detected. By switching or modulating thevariable current source 7 information can be transmitted back to theinterrogating source.

FIG. 4 shows a circuit arrangement typically for use at UHF frequencies.The circuit is generally similar to that shown schematically in FIG. 1and the same parts are identified with the same reference numerals. Thematching network 3 is provided by a capacitor C₂ and a choke RFC₁. Thebias network 5 is provided by a capacitor C₃. Termination networks 11,12 are constituted by a capacitor C₄ and resistor R₁. The bias network 6on the output side is provided by a further choke RFC₂ and resistor R₂connected in series.

I claim:
 1. A modulating reflector circuit for reflecting and modulatingincoming signals from a signal source, comprising:a) a transistor havinginput and output terminals connected in a feedback path, said transistorhaving a current/voltage operating characteristic having a generallylinear region; b) biasing means for biasing the transistor to operate insaid region to reflect the incoming signals with an increased magnitudein a reflect mode; and c) modulating means for modulating electricalpower to the transistor to modulate the reflected signals, saidmodulating means being operable to modulate the electrical power suchthat the reflected modulated signal is a substantially single sidebandsignal.
 2. The reflector circuit, according to claim 1, wherein themodulating means modulates the electrical power supplied to thetransistor with a periodic waveform, and wherein the periodic waveformhas a time period whose reciprocal is the frequency of said singlesideband signal.
 3. The reflector circuit, according to claim 2, whereinthe periodic waveform is a distorted sine wave.
 4. The reflectorcircuit, according to claim 1, wherein the transistor is a field effecttransistor whose input terminal is a gate and having output terminalswhich are a source and a drain, respectively.
 5. The reflector circuit,according to claim 4, wherein the incoming signals are at microwavefrequencies, and wherein the field effect transistor has intrinsiccapacitance between the gate and the source, said intrinsic capacitanceserving as the feedback path at said microwave frequencies.
 6. Thereflector circuit, according to claim 4, and further comprising adiscrete capacitor connected in a conductive line between the gate andthe transistor source.
 7. The reflector circuit, according to claim 1,and further comprising an antenna coupled to the input terminal of thetransistor for transmitting the incoming signals to the input terminalof said transistor.
 8. The reflector circuit, according to claim 7, andfurther comprising a matching network connected in a conductive linebetween the antenna and the input terminal of said transistor.
 9. Thereflector circuit, according to claim 1, and further comprisingterminations connected to the output terminals of said transistor. 10.The reflector circuit, according to claim 1, and further comprisingcontrol means for selectively biasing the transistor to operate in anon-linear relatively lower gain region of the transistor'scurrent/voltage operating characteristic to that of said generallylinear region, such that the transistor operates as a detector to detectthe incoming signals and any modulation of the signals thereof in adetect mode.
 11. The reflector circuit, according to claim 10, whereinthe transistor is a field effect transistor whose input terminal is agate and having output terminals which are a source and a drainrespectively, and wherein the control means includes a variable currentsource connected to the transistor source and drain, and wherein thecontrol means is operative to switch the transistor between said detectand reflect modes by varying electrical current flowing between thetransistor source and drain.
 12. The reflector circuit, according toclaim 1, wherein the transistor is a field effect transistor whose inputterminal is a gate and having output terminals which are a source and adrain respectively, and wherein the modulating means is operative tomodulate the magnitude of the electrical current flowing between thetransistor source and drain.
 13. The reflector circuit, according toclaim 1, and further comprising control means for selectively biasingthe transistor to operate in a relatively higher current region of saidoperating characteristic in an unstable mode in which the transistoroscillates at an oscillation frequency.
 14. The reflector circuit,according to claim 13, wherein the control means includes a variablecurrent source for supplying electrical current to the transistor, andwherein the control means is operative for switching the current sourceon and off; and further comprising means for detecting an incomingsignal having a frequency substantially identical to said oscillationfrequency upon decreasing the time taken for the transistor to reach apredetermined level of oscillation.
 15. The reflector circuit, accordingto claim 1, wherein the reflector circuit is used with a transponder forprocessing amplitude modulated incoming signals.